In the field of medical electronics there is a continual need for more reliable measurements of the breathing rate of a human being. The present invention provides means for reliably measuring human respiration. It monitors and processes the change in thoracic impedance to provide breathing rate information. This monitor includes circuitry to prevent the cardiac activity of the patient from being counted as respiratory activity and falsely resetting an apnea timer or the like.
Monitors of this kind monitor the respiration frequency of the patient, record the respiration curve, and indicate respiration irregularities such as apnea. Impedance variations in the thorax region caused by respiration activity are obscured by interferring influences which are primarily introduced by heart activity. This interference is usually referred to as cardio-vascular artifact, hereinafter referred to as CVA. Both respiration activity and heart activity result in a periodic change in the thorax impedance. However, the amplitude of the variation caused by the heart activity is substantially smaller than the variation caused by respiration and the heart beat is usually higher in frequency than the respiration.
In order to suppress disturbances, some known monitors feed the electrical signals obtained by a variation of the thorax impedance to a trigger circuit, which will only deliver an output signal when the amplitude of the supplied input signal exceeds a predetermined threshold value. This threshold value is manually adjustable and is selected so that it is lower than the amplitude of the signals produced by the respiration activity and so that it is higher than the amplitude of the signals caused by the heart activity. One disadvantage of this type of respiration monitor is that the threshold value has to be readjusted frequently as the amplitudes of the respiration signals do not only differ from patient to patient but may also differ with the same patient over an extended period of time. Another difficulty is that the threshold value cannot be adjusted accurately since the periodic impedance variations due to heart activity are generally exceeded by those caused by respiration activity.
In order to avoid such manual readjustment of the trigger threshold, another known type of respiration monitor is provided with a trigger level controller. The controller automatically adjusts the threshold value to a certain fraction, for example to two thirds, of the actual amplitude of the respiration signal. The readjustment occurs with a certain delay so that it will be primarily influenced by respiration signals having a high amplitude, while it tends not to be influenced by interferring signals which occur between those high amplitude signals.
Furthermore, a lower limit is provided for the threshold value, which is higher than the lowest amplitudes of the respiration signals. This lower limit, however, should be higher than the highest possible amplitude of the heart beat signals. In practice, these two requirements cannot be met simultaneously, as the amplitude of the respiration signals may be equal or smaller than that of the signals introduced by the heart activity. If the lower limit of the threshold value is made so high that it is above the amplitude of the heart signals in all cases, it may happen that the respiration monitor does not respond to weak respiration signals. If the lower limit for the threshold value is low enough for weak respiration signals, the automatic readjustment may fail if apnea occurs or if the amplitude of the respiration signals is not substantially higher than that of the heart signals. In these cases, there will result a threshold value which has a lower amplitude than the heart signals. Consequently, the trigger circuit will supply output signals which are caused by heart activity and which will, thus, result in wrong indication of the respiration activity.